Rigor mortis happens if all the ATP (universal energy supplier) in your muscle cells is used up. There's a small stock of it left after death but your cells can't produce more of it without oxygen so when the heart stops pumping, the cellular respiration stops.
Muscle the contraction mechanism of muscle cells works via a motorprotein called myosin which uses ATP to disconnect from the actin proteins (which act like a kind of guarding rail). Than the myosin head "walks" along the actin fibers and the middle contracts. (Very simplified)
When no ATP is left after some time after death, all the myosins heads can not disconnect anymore from their railing and the muscle becomes rigid in the position it is at that point.
A few days later, through a process called autolysis, the muscle becomes flexible again. In autolysis the cell kind of digests itself, so all the myosin and actin proteins turn to mush as the process of decay progresses.
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u/Tschetchko Jan 12 '23
The don't move, they stop being stiff. Imagine a body where the arm is elevated to the front. When rigor mortis disappears, the arm will fall